In these days, nucleic acids (DNA=deoxyribonucleic acid, RNA=ribonucleic acid) are subject to various analyses and assays in clinical analysis and medical diagnostics. Since the initial amount of nucleic acids normally is very low, nucleic acids have to be amplified prior to their use so as to obtain sufficient amounts which can be used as starting material.
The amplification of nucleic acids using the well-known polymerase chain reaction (PCR) has been extensively described in the patent literature, for instance, in U.S. Pat. Nos. 468,203, 4,683,195, 4,800,159 and 4,965,188. Generally, in the polymerase chain reaction, samples containing reaction mixtures of specific reagents and nucleic acids are repeatedly put through a sequence of amplification steps. Each sequence includes a step of melting the double-stranded nucleic acids to obtain denaturated single polynucleotide strands, a step of annealing short primers to the strands and a step of extending those primers to synthesize new polynucleotide strands along the denaturated strands to make new copies of double-stranded nucleic acids. Due to the fact that reaction conditions strongly vary with temperatures, the samples are put through a series of temperature excursions in which predetermined temperatures are kept constant for specific time intervals (“thermo-cycling”). The temperature of the samples typically is raised to around 90° C. for melting the nucleic acids and lowered to a temperature in the range of from 40° C. to 70° C. for annealing and primer extension along the polynucleotide strands. It is known to detect the reaction products even during the progress of the polymerase chain reaction (“real-time PCR”) to thereby obtain more information about the amplification process and to improve the reliability of the detection results.
In daily routine, the PCR is performed in commercially available instruments enabling a large number of reaction mixtures to be cycled simultaneously. Usually, integrally molded plastic disposables provided with plural open-top wells sized to receive the reaction mixtures are used for nucleic acid amplification. Such disposables are commonly known as “microplates”.
It has been found advantageous to provide the open-top wells with a sealing cover for air-tightly sealing individual wells containing the reaction mixtures. One reason is the necessity to avoid evaporation of liquids so as to ensure the integrity of the reaction mixtures. Another reason is to prevent spilling of the contents of the wells during transport of the microplate from one location to another. A yet another reason is to prevent cross contamination of individual reaction mixtures contained in the wells so as to provide a generally sterile and controlled environment under which the amplification steps can be carried out.
It is convenient to use transparent sealing covers such as thin plastic foils applied to the top surface of the microplate which allow for an optical detection of the reaction products even during progress of the reactions. In practical use, for instance, an adhesive plastic foil provided with an adhesive backing is positioned over the microplate so that the adhesive backing faces the upper surface of the microplate. The plastic foil then is pressed on the upper surface, e.g., by means of a pressure roll rolling back and forth to thereby obtain uniform adhesion of the sealing foil to the microplate. Adhesive foils, however, often cause problems with respect to an air-tight sealing of individual wells. Accordingly, undesired evaporation of fluids impairing the reproducibility of test results especially in the case of small sample volumes and cross contamination between various reaction mixtures may occur. Otherwise, the adhesive material may probably influence the outcome of the nucleic acid amplification steps thus downgrading the reliability of the test results.
Better results can normally be obtained using thermally fusible foils. In practical use, the foil is positioned over the microplate and heated, e.g., by means of a heated sealing stamp which can be brought in and out of contact with the foil. While heated, the sealing foil is pressed onto the microplate to ensure a close adhesive fit with full contact to the microplate.
In light of the foregoing, it is an object of the invention to provide an improved instrument and method for automatically heat-sealing microplates. It is a further object of the invention to provide an improved system for processing, especially thermally processing, and/or analyzing liquid samples. These and further objects are met by an instrument and method for thermally heat-sealing microplates as well as a system for thermally processing liquid samples according to the independent claims. Preferred embodiments of the invention are given by the features of the dependent claims.